Rain Ratio Research Articles

Cenozoic pelagic Sr/Ca records: Exploring a link to paleoproductivity

Recent studies have revealed that Sr/Ca ratios of coccolithophores may be affected by productivity. Here we compile published Sr/Ca data from bulk carbonate, fine fraction sediment, and planktonic foraminiferal Sr/Ca records that span the past 60 Myr and attempt to place these records into a paleoceanographic framework. We account for changes in seawater Sr/Ca ratios using the curve of Lear et al. [2003] and discuss our observations with respect to changes in the partitioning coefficient of Sr through time. We discuss limitations associated with postburial processes, temporal changes in the coccolith/planktonic foraminiferal rain ratio, and the coccolith species composition of the sediment. Despite these caveats, we are able to show that in the bulk and fine fraction carbonate records there are two broad periods of enhanced partitioning of Sr relative to today, the Oligocene, and the middle to late Miocene/early Pliocene. Because these are two intervals for which we can cite evidence for a relatively productive ocean on the regional or global scale, we believe that the Sr/Ca ratios of bulk carbonate can be explained, at least in part, by the effects of oceanic nutrient levels on Sr uptake during calcification of coccoliths, which make up the vast majority of these sediments. Within the limits of the inferred seawater Sr/Ca record the results of this study contribute a geologic perspective to recent laboratory and field studies that have raised the possibility that Sr incorporation into biogenic calcite is controlled by biogeochemical processes.

Dust‐induced changes in phytoplankton composition in the Tasman Sea during the last four glacial cycles

An increase in iron supply associated with enhanced dust inputs could be responsible for higher marine phytoplankton production leading to the typically lower glacial atmospheric CO2 concentrations, as suggested by the “iron hypothesis.” The enhanced dust supply may also have provided the oceans with significant amounts of silica, which would have favored the growth of diatoms over coccolithophores, as suggested by the “silica hypothesis.” Here we present new data on molecular biomarkers in a sediment core from the midlatitudes of the Southern Hemisphere, which reveal dust‐induced changes in the relative contribution of the phytoplankton to total productivity. Our results illustrate a shift in the relative abundance of siliceous over calcareous organisms during glacial times, when terrestrial aeolian input was enhanced. Although we did not detect a significant glacial decrease in coccolithophorid productivity, the decrease in the CaCO3/Corg rain ratio could have still contributed to some extent in lowering atmospheric CO2 levels.

More on Isotopic Analysis of Teeth and Bones

Wright comments: Many thanks to Turgut Dincer for his comments and corrections on Charles Day’s article, which reported our work on strontium and oxygen isotopes in ancient Maya skeletons. Dincer is correct; the paragraph describing oxygen isotopes in rainfall was poorly stated. In communications with Day, I suspect that I described the changes in the oxygen isotope ratio of rain across the Maya area in exactly the same manner as Dincer did in his final sentence. The patterns we see are a function of elevation, relative humidity, and the path of rainstorms across the landscape. Unfortunately, I did not catch this inaccuracy in a quick prepress reading of the article. Nonetheless, Dincer’s correction does not change in any way our interpretations of the data nor the implications of our work for Maya prehistory.© 2004 American Institute of Physics.

Isotopic fractionation of water during evaporation

Variations in the isotopic content (18O/16O and D/H ratios) of water in the natural environment provide a valuable tracer of the present‐day global hydrologic cycle and a record of the climate over at least 400,000 years that is preserved in glacial ice. The interpretation of observed isotopic ratios in water vapor, rain, snow, and ice depends on our understanding of the processes (mainly phase changes) that produce isotopic fractionation. Whereas equilibrium isotopic fractionation is well understood, kinetic effects, or diffusion‐controlled fractionation, has a limited experimental foundation. Kinetic effects are significant during evaporation into unsaturated air and during condensation to form ice from vapor. Kinetic effects are also thought to control the deuterium excess (d = δD − 8δ18O) of precipitation. We describe experiments to observe kinetic effects associated with evaporation. Analysis of our own and previous experiments shows that surface cooling of the liquid is a crucial variable affecting fractionation from evaporating water that has not been properly considered before. Including the effects of evaporative surface cooling reconciles observed D/H fractionation with kinetic theory and removes the need to invoke an unusual size for the HDO molecule. Thus the isotopic molecular diffusivity ratios are D(H218O)/D(H216O) = 0.9691 and D(HD16O)/D(H216O) = 0.9839. Implications of this work for representation of kinetic fractionation in global circulation models and cloud physics models are briefly discussed.

Temporal variability of rainwater iron speciation at the Bermuda Atlantic Time Series Station

Diurnal and seasonal variations in rainwater iron speciation were investigated during a summer and winter cruise aboard the RV Endeavor at the Bermuda Atlantic Time Series Station. Concentrations of total Fe, Fe(II), and Fe(III) were all higher in rainwater falling at BATS during the summer relative to winter cruise. Iron speciation displayed marked diel variability with a 200‐fold higher Fe(II)(aq)/Fe(III)(aq) ratio in rain received between 1200 and 1800 local time (LT) relative to late evening or early morning summer rain. The higher Fe(II)/Fe(III) ratio in the afternoon indicates Fe(II) is produced at the expense of Fe(III) during the course of the day, suggesting photochemical processes are important in controlling iron speciation in marine rains falling over the open ocean. Experiments utilizing authentic rainwater collected at BATS during both summer and winter cruises demonstrate that rainwater Fe(II)(aq) is stabilized against oxidation and precipitation for more than 4 hours after mixing with BATS seawater. Preliminary results suggest this stabilization is the result of organic complexation of Fe in rainwater. Owing to the isolation of BATS from riverine sources of Fe, rainwater is the predominant source of soluble and stable Fe(II) in this region of the North Atlantic.

Seasonal characteristics of precipitation in 1998 over East Asia as derived from TRMM PR

Precipitation radar data derived from the Tropical Rainfall Measuring Mission (TRMM) satellite are used to study precipitation characteristics in 1998 over East Asia (10°–38°N, 100°–145°E), especially over mid-latitude land (continental land) and ocean (East China Sea and South China Sea). Results are compared with precipitations in the tropics. Yearly statistics show dominant stratiform rain events over East Asia (about 83.7% by area fraction) contributing to 50% of the total precipitation. Deep convective rains contribute 48% to the total precipitation with a 13.7% area fraction. The statistics also show the unimportance of warm convective rain in East Asia, contributing 1.5% to the total precipitation with a 2.7% area fraction. On a seasonal scale, the results indicate that the rainfall ratio of stratiform rain to deep convective rain is proportional to their rainfall pixel ratio. Seasonal precipitation patterns compare well between Global Precipitation Climatology Project rainfall and TRMM PR measurements except in summer. Studies indicate a clear opposite shift of rainfall amount and events between deep convective and stratiform rains in the meridional in East Asia, which corresponds to the alternative activities of summer monsoon and winter monsoon in the region. The vertical structures of precipitation also exhibit strong seasonal variability in precipitation Contoured Rainrate by Altitude Diagrams (CRADs) and mean profiles in the mid-latitudes of East Asia. However, these structures in the South China Sea are of a tropical type except in winter. The analysis of CRADs reveals a wide range of surface rainfall rates for most deep convective rains, especially in the continental land, and light rain rate for most stratiform rains in East Asia, regardless of over land or ocean.

An end to the “rain ratio” reign?

One of the most elegant mechanisms forwarded for late Quaternary atmospheric CO2 variability concerns the sensitivity of calcium carbonate preservation in deep ocean sediments to the relative delivery rates of calcium carbonate and particulate organic carbon (the CaCO3:POC “rain ratio”). It was implicitly assumed that any change in the CaCO3:POC rain ratio of biogenic material produced in the surface ocean will be communicated directly to the sediments. This would allow relatively subtle shifts in ecosystem composition to affect sedimentary CaCO3 preservation (and thus atmospheric CO2). However, recent research into the controls on the transport of POC to depth suggests that the rain ratio “seen” by the sediments may instead be buffered against any perturbation occurring at the surface. This casts doubt on the viability of hypotheses envisaging ecological changes as a means of accounting for the observed glacial‐interglacial CO2 signal.

Climate warming could reduce runoff significantly in New England, USA

The relation between mean annual temperature (MAT), mean annual precipitation (MAP) and evapotranspiration (ET) for 38 forested watersheds was determined to evaluate the potential increase in ET and resulting decrease in stream runoff that could occur following climate change and lengthening of the growing season. The watersheds were all predominantly forested and were located in eastern North America, along a gradient in MAT from 3.5 °C in New Brunswick, CA, to 19.8 °C in northern Florida. Regression analysis for MAT versus ET indicated that along this gradient ET increased at a rate of 2.85 cm °C −1 increase in MAT (±0.96 cm °C −1, 95% confidence limits). General circulation models (GCM) using current mid-range emission scenarios project global MAT to increase by about 3 °C during the 21st century. The inferred, potential, reduction in annual runoff associated with a 3 °C increase in MAT for a representative small coastal basin and an inland mountainous basin in New England would be 11–13%. Percentage reductions in average daily runoff could be substantially larger during the months of lowest flows (July–September). The largest absolute reductions in runoff are likely to be during April and May with smaller reduction in the fall. This seasonal pattern of reduction in runoff is consistent with lengthening of the growing season and an increase in the ratio of rain to snow. Future increases in water use efficiency (WUE), precipitation, and cloudiness could mitigate part or all of this reduction in runoff but the full effects of changing climate on WUE remain quite uncertain as do future trends in precipitation and cloudiness.

Productivity and organic carbon rain to the California margin seafloor: Modern and paleoceanographic perspectives

We have developed a proxy measurement for the determination of organic carbon (Corg) oxidation rates (Cox) on the seafloor utilizing the δ13C values of two benthic foraminifera: Bolivina argentea, a near‐surface dwelling species, and Buliminella tenuata, an infaunal species. The δ13C isotopic composition of these tests preserve a record of the Σ13CO2 pore water gradient, which is directly related to Cox. On the seafloor of Santa Monica Basin, Cox has fluctuated between 0.6 and 2.5 mmolC m−2 d−1 and generally increased from 1600 to 1980 AD. Between 1920 and 1970, Cox increased from 1 to 2 mmolC m−2 d−1 and then decreased into the 1980s. By adding Cox to a high‐resolution record of Corg burial rate, we have derived the pattern of Corg rain to the seafloor, which fluctuated between 2.9 and 3.8 mmolC m−2 d−1 over the last 80 years. The ratio of Corg rain at 900 m to primary productivity is constant at 10 ± 1% over a 40% change in mean annual primary productivity. Sediments in Santa Monica Basin accumulate ∼50% of the Corg raining to the seafloor; however, there is a trend toward lower burial efficiency with higher productivity and rain rate.

Association of sinking organic matter with various types of mineral ballast in the deep sea: Implications for the rain ratio

We compiled and standardized sediment trap data below 1000 m depth from 52 locations around the globe to infer the implications of the Armstrong et al. [2002] “ballast” model to the ratio of organic carbon to calcium carbonate in the deep sea (the rain ratio). We distinguished three forms of mineral ballast: calcium carbonate, opal, and lithogenic material. We concur with Armstrong et al. [2002] that organic carbon sinking fluxes correlate tightly with mineral fluxes. Based on the correlations seen in the trap data, we conclude that most of the organic carbon rain in the deep sea is carried by calcium carbonate, because it is denser than opal and more abundant than terrigenous material. This analysis explains the constancy of the organic carbon to calcium carbonate rain ratio in the deep sea today, and argues against large changes in the mean value of this ratio in the past. However, sediment trap data show variability in the ratio in areas of high relative calcium carbonate export (mass CaCO3/mass ratio > 0.4), unexplainable by the model, leaving open the possibility of regional variations in the rain ratio in the past.

The influence of particle composition and particle flux on scavenging of Th, Pa and Be in the ocean

We have examined the relative affinity of Th, Pa and Be for sorption from seawater onto particles of variable composition (opal, carbonate, lithogenic particles and organic carbon). Nuclide concentrations in particles collected from time-series sediment traps were normalized by the dissolved nuclide concentration in the overlying water column in order to compute partition coefficients under conditions spanning a wide range of particle flux and particle composition. Our results suggest that the affinity of particles for Pa and Be increases with their increasing opal content and decreasing carbonate content, while the affinity of particles for Th increases with increasing carbonate content, and decreases with increasing opal content. We find no correlation between the aluminosilicate content of particles and their affinity for scavenging of any of these elements. Extrapolating to a pure CaCO 3 end member, the partition coefficient for Th (9.0×10 6 g g −1) is ∼40 times greater than for Pa, and roughly 100 times greater than for Be, whereas for a pure opal end member, the partition coefficient for Th (3.9×10 5 g g −1) is slightly less than that for Pa and Be. Partition coefficients decrease with increasing particle flux in open-ocean settings, but not in an ocean-margin region. This kinetic effect reflects the increasing contribution of unaltered surface material reaching the sediment traps as particle flux increases. The degree of fractionation between Pa and Th and between Be and Th depends on the opal:carbonate rain ratio. These results challenge the use of sedimentary 231Pa/ 230Th and 10Be/ 230Th ratios as simple proxies of particle flux. However, the strong dependence of nuclide scavenging on the opal:carbonate rain ratio may provide a needed tool for reconstructing past changes in planktonic community composition.

Silicic acid leakage from the Southern Ocean: A possible explanation for glacial atmospheric pCO2

Using a simple box model, we investigate the effects of a reduced Si:N uptake ratio by Antarctic phytoplankton on the marine silica cycle and atmospheric pCO2. Recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added [Hutchins and Bruland, 1998; Takeda, 1998; Franck et al., 2000]. The Southern Ocean may have supported diatoms with reduced Si:N uptake ratios compared to today during the dustier glacial times [Petit et al., 1999]. A similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as Phaeocystis. Our model shows that reduced Si:N export ratios in the Southern Ocean create excess silicic acid, which may then be leaked out to lower latitudes. Any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes CaCO3 production and therefore diminishes the carbonate pump. In our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric CO2 from the interglacial 277 to 230–242 ppm, depending on where the excess silicic acid is consumed. By comparison, the atmospheric pCO2 sensitivity of general circulation models to carbonate pump forcing is ∼3.5–fold greater, which, combined with carbonate compensation, can account for peak glacial atmospheric pCO2. We discuss the importance of the initial rain ratio of CaCO3 to organic carbon on atmospheric pCO2 and relevant sedimentary records that support and constrain this “silicic acid leakage” scenario.

Spectral representation of rain profiles and diurnal variations observed with TRMM PR over the equatorial area

A summary of rainfall statistics over the equatorial area (10°N–10°S), observed by a space‐borne precipitation radar of the Tropical Rainfall Measuring Mission (TRMM), is presented utilizing all nadir data of PR2a25 version 5 for the period of 1998–1999. Spectral representation of rain profiles is introduced, and convective and stratiform rain characteristics over land and over ocean are examined. With 0.3 mm hr−1 rain‐top threshold, convective: stratiform rain ratios at 2–4 km over land and over ocean are, 61:39 and 50:50 in amount, 20:80 and 17:83 in area, respectively. For diurnal variations over ocean, two types of rain vary almost synchronously, with an early‐morning maximum in 03–06 LT, indicating that direct radiative effect is the dominant diurnal control. Over land, convective rain has a distinct maximum of afternoon showers in 15–18 LT, while stratiform rain has a midnight (24–03LT) maximum. Also shown is that afternoon showers are more frequent but nighttime rain is stronger, suggesting contributions from large‐scale disturbances.

Delineating the karstic flow system in the upper Lost River drainage basin, south central Indiana: using sulphate and δ34S SO 4 as tracers

A karstic flow system in the upper Lost River drainage basin in south central Indiana, USA, was investigated using SO 4 concentration and δ 34S SO 4 as tracers. The flow system was characterized as vadose flow and phreatic diffuse flow. Vadose-flow samples were collected from 7 epikarstic outlets after storm events. Phreatic diffuse flow samples were collected from the Orangeville Rise, the major emergence point for the drainage basin, during the base flow periods. Discharge from the Orangeville Rise was constant during the base flow periods but showed large variations in flow rate (0.3–11.7 m 3/s), SO 4 concentration (11–220 mg/l), and δ 34S SO 4 (+5.2 to +15.0‰) after storm events, due to the mixing of rain, vadose flow, and phreatic diffuse flow in the conduits that feed the Orangeville Rise. Sulphate concentrations and δ 34S SO 4 were unique in vadose flow (S SO 4 : 13–24 mg/l; δ 34S SO 4 : +1.9 to +3.8‰) and phreatic diffuse flow (SO 4: 220 mg/l; δ 34S SO 4 : +15.0‰). Mean SO 4 concentration of rainwater in the study area was measured as 1.8 mg/l. Using a 3-component mixing model for water in the karstic conduits, the mixing ratios of rain (16.5%), vadose flow (58.5%), and phreatic diffuse flow (25.0%) components were calculated in the Orangeville Rise discharge. These mixing ratios attained using SO 4 concentration as a tracer indicated the important role of the vadose zone as a water storage area in karst aquifers.

Flux of particulate inorganic carbon to the deep subarctic Pacific correlates with El Niño

Time-series data are presented from sequentially operating deep (3800 m) sediment traps deployed continuously in the northeastern subarctic Pacific between 1982 and 1990. These data show that monthly anomalies in the fluxes of particulate organic carbon (POC) and particulate inorganic carbon (PIC) were significantly correlated with equatorial surface-water temperature anomaly (Niño 3.4 index). This “teleconnection” was stronger for PIC than for POC, and average PIC flux almost doubled from 3.60 to 6.41 mg C m−2 d−1 during the years influenced by El Niño. Average POC flux increased from 2.28 to 3.18 mg C m−1 d−1 during these same years influenced by El Niño, although interpretation of this average was complicated by a single high-flux event in August 1983. Rain ratio (mean daily PIC:POC flux) was ≈25% higher (increased from 1.70 to 2.17) during the years influenced by El Niño. Oceanic sequestration of atmospheric CO2 is particularly sensitive to the rain ratio in the long term, and a decrease in PIC deposition of only 40% has been invoked as sufficient to decrease atmospheric CO2 to the glacial value. An improved mechanistic understanding of the primary formation and sinking of PIC should help to unravel some of the poorly understood biogeochemical feedbacks operating on ocean/atmosphere exchange of CO2 over contemporary and geological time scales.

Assessing the importance of the Southern Ocean for natural atmospheric pCO2 variations with a global biogeochemical general circulation model

Abstract A global biogeochemical ocean general circulation model (the HAMOCC model) is employed to study the efficiency of the Southern Ocean for enhancing or damping atmospheric p CO 2 variations due to (1) internal and (2) external perturbations of the carbon cycle. Internal perturbations are changes of ocean parameters that govern the carbon cycle such as the velocity field, seawater temperature, the uptake rate of carbon and nutrients, and the rain ratio of C(POC):C(CaCO 3 ) in biogenic particle export from the euphotic zone. The model results suggest that the Southern Ocean is more efficient for compensating external p CO 2 variations than the North Atlantic. Concerning most biogeochemical parameters associated with the “biological carbon pump” (POC pump and CaCO 3 counter pump), the Southern Ocean enhances natural p CO 2 variations and dampens those for changes in sea-surface temperature or circulation. Therefore, modifications in the Southern Ocean biogenic carbon pumps may play a key role for natural variations of the atmospheric CO 2 content.

Late Quaternary variations in calcium carbonate preservation of deep-sea sediments in the northern Cape Basin: results from a multiproxy approach

In this study, we test various parameters in deep-sea sediments (bulk sediment parameters and changes in microfossil abundances and preservation character) which are generally accepted as indicators of calcium carbonate dissolution. We investigate sediment material from station GeoB 1710-3 in the northern Cape Basin (eastern South Atlantic), 280 km away from the Namibian coast, well outside today’s coastal upwelling. As northern Benguela upwelling cells were displaced westward and periodically preceded the core location during the past 245 kyr (Volbers et al., submitted), GeoB 1710-3 sediments reflect these changes in upwelling productivity. Results of the most commonly used calcium carbonate dissolution proxies do not only monitor dissolution within these calcareous sediments but also reflect changes in upwelling intensity. Accordingly, these conventional proxy parameters misrepresent, to some extent, the extent of calcium carbonate dissolution. These results were verified by an independent dissolution proxy, the Globigerina bulloides dissolution index (BDX′) (Volbers and Henrich, submitted). The BDX′ is based on scanning electronic microscope ultrastructural investigation of planktonic foraminiferal tests and indicates persistent good carbonate preservation throughout the past 245 kyr, with the exception of one pronounced dissolution event at early oxygen isotopic stage (OIS) 6. The early OIS 6 is characterized by calcium carbonate contents, sand contents, and planktonic foraminiferal concentrations all at their lowest levels for the last 245 kyr. At the same time, the ratio of radiolarian to planktonic foraminiferal abundances and the ratio of benthic to planktonic foraminiferal tests are strongly increased, as are the rain ratio, the fragmentation index, and the BDX′. The sedimentary calcite lysocline rose above the core position and GeoB 1710-3 sediments were heavily altered, as attested to by the unusual accumulation of pellets, aggregates, sponge spicules, radiolaria, benthic foraminifera, and planktonic foraminiferal assemblages. Solely the early OIS 6 dissolution event altered the coarse fraction intensely, and is therefore reflected by all conventional calcium carbonate preservation proxies and the BDX′. We attribute the more than 1000 m rise of the sedimentary calcite lysocline to the combination of two processes: (a) a prominent change in the deep-water mass distribution within the South Atlantic and (b) intense degradation of organic material within the sediment (preserved as maximum total organic carbon content) creating microenvironments favorable for calcium carbonate dissolution.

Seasonal dependence of the chiral composition of α-HCH in coastal deposition at the North Sea

The environmental pollutant α-HCH was analysed in seawater, air and bulk deposition samples from the North Sea. Enantioselective analyses of the sample extracts gave evidence of the change of the direction of net air–water gas transfer of the contaminant in dependence on the season. Due to warmer water surface temperatures during late summer to early autumn the equilibrium of α-HCH between air and water is dominated rather by volatilisation than deposition. The volatilisation of non-racemic α-HCH, as known to occur from seawater, changes the enantiomeric ratio in air, which is reflected in the observed ratio in rain that passes the air column.

Sr Isotopic Signature in Plant-Derived Ca in Rain

The 87Sr/86Sr ratios in rain at three sites in Japan were about 0.706 to 0.712, whereas those at one site in central Korea were higher (0.711 to 0.716), reflecting the difference in geology between the two countries. Because the 87Sr/86Sr ratios of exchangeable sites in soil minerals are indistinguishable from those of associated plants, aerosol Sr originating from both can be grouped together as plant-derived Sr. Spatial and temporal variations in 87Sr/86Sr and Sr/Ca ratios in the rain suggest the presence of plant-derived Sr and Ca in addition to Sr and Ca derived from sea-salt, acid-soluble aeolian carbonate from China, and cement-derived carbonate. However, systematic data on the 87Sr/86Sr ratios and elemental concentrations of soils, plants, fly-ash, and road dust are required to put reliable constraints on the provenance of atmospheric Ca.

Geochemical Comparison of Stream Water, Rain Water, and Watershed Geology in Central Korea

A hilly to mountainous watershed in Chonju in central Korea does not receive acid rain (average pH: 6.2); however, the stream water in the granite watershed is slightly acidic (6.4–6.7) and contains a low concentration of Ca compared to the stream water in sedimentary and volcanic rock watersheds (6.8–7.6). Although the concentrations of Ca and Sr and the 87Sr/86Sr ratio in the stream water change in accordance with the watershed geology, the stream 87Sr/86Sr ratios are closer to the 87Sr/86Sr ratios of rain than to those of the substrate rocks, suggesting the selective but sluggish weathering of Ca-containing minerals neutralizes acid. The concentrations of trace metals (As, Cr, Cu, Mo, Ni, Pb) in the water are lower than those in rain and less dependent on the watershed geology, indicating that they originated dominantly from the atmosphere. This result is consistent with the stream water having Pb isotope ratios close to that of rain but distinct from that of the rocks. We assume that the soil pool of exchangeable ions dominantly contains atmospherically derived heavy metals, which are subsequently discharged into streams. It is likely that the poor acid-neutralizing capacity of granite makes the aquatic systems in the granite watershed in Chonju sensitive to atmospheric acidification.